re: Why throw the ball with a first and goal from the 2 yard line?

it was a perfect play call. Just poor/unfortunate execution.
Recall that this was about the fourth consecuctive play from inside the 10 yard line (AU had 2 PI penalties) Teh couple of other run attempts were jammed at the LOS.
This play, if nothing else, was set up on 1st down to open up the defense.
The biggest mistake made in the play was Kaho (normally a LB) was lined up at FB and totally ignored the block on the charging DE for AU. Had he even just rubbed him, Mac gets around him and cake walks into the endzone. Or he could have thrown it to Najee who had his man beat (thus how the luck of his getting that INT) by two or three steps. Worst case was an incomplete pass. But when Kaho ignored the block, Mac should have just eaten the ball. But to his defense, even in that split second he could see Najee was wide open.